1. Field of the Invention
This invention relates to inks for use in flexographic printing. More particularly, this invention relates to solvent-free, solid inks for use in hot melt flexographic printing presses.
2. Description of Related Art
Conventional flexographic printing inks typically require organic solvents or water for their application. The use of solvents is well known to be linked with a variety of disadvantages, both for the manufacturer as well as the printer, for example the danger of fire, explosion, odor, and environmental pollution. Such inks, in addition to these disadvantages, also lead to a lessening of the printing quality due to residual solvent, for example a decrease in the blocking temperature. Also, only certain maximum printing speeds can be achieved which are determined by the rate of evaporation of the solvent or water employed, and which cannot be sufficiently increased by printing press construction.
The problems of environmental pollution by solvents have led to the development of solvent-free printing inks which contain reactive components which are hardened at elevated temperatures or by incident radiation. In particular, reactive monomers and/or oligomers have been used as vehicles for printing inks. Such reactive inks when printed are hardened on a substrate by heat, ultraviolet light or electron radiation in a very short time to form a film. While such solvent-free inks have eliminated pollution by solvents the hardened printed inks frequently contain residual unreacted monomers which can lead to contamination in such applications as food packaging.
Hot melt inks have been under development for several years. Illustrative of such development are the gravure and flexographic printing processes employing solvent-free inks, solid at room temperature but molten at printing temperatures, which have been disclosed in U.S. Pat. No. 4,066,585. The disclosed solid inks comprise a pigment and a thermoplastic binder having a softening point between 90° C. and 160° C. The binder comprises a synthetic polyamide resin or synthetic polyesteramide resin, each resin being the condensation product of (1) an acid component comprising a dimerized fatty acid and a monocarboxylic acid and (2) an amine component comprising a diamine and, in the case of the polyesteramide resin, additionally comprising a diol and/or alkanolamine.
While advances have been made in hot melt ink technology, there remain major problems with its efficient use in the industry. One such problem confronting this technology in gravure printing is the remelting of inks in succeeding print stations. In gravure printing, the previously printed ink comes in contact with the non-image area of the cylinder of the next print station; thus, allowing transfer to occur if the temperature of the cylinder and press speed are such to allow the ink to melt. Flexographic printing overcomes the remelting problem because ink is transferred to the paper/substrate from raised image rather than from an etched gravure cell. In the flexographic printing process, ink on the print from the previous station therefore does not come in contact with the non-image area of the plate. Another problem confronting this technology is the elimination of volatile components, particularly at the high temperatures used on printing press. In view of the present state of the technology, there continues to be an industry need for hot melt inks that can be used with flexographic printing plates to produce high quality printing on various substrates.